1. Field of the Invention
The present invention relates to a glass optical element. Particularly, the invention relates to a beam shaping element that converts oval output light from a blue laser diode or the like into circular light.
2. Description of the Related Art
In general, a light source which is used in a pickup optical system is a laser diode (LD), its output light is an oval divergent beam. When the divergent beam is focused by an objective lens directly, the beam is emitted only to a part of a circular recording region or also to the outside of the recording region, thereby deteriorating accuracy of recording and reproduction. A sectional shape of the beam should be, therefore, shaped so that the section has a circular shape on a recording medium.
Particularly in recent years, although blue semiconductor lasers are used as light sources, the accuracy required for recording and reproducing signals becomes strict due to shortening of a wavelength. Output of the blue lasers is, however, weak at present, a sufficient laser power for accurate recording and reproducing cannot be secured. In order to solve this problem, utilization efficiency of a laser should be heightened by shaping the oval section of a beam from LD into a circular beam section. For this reason, a beam shaping technique becomes very important.
Beams are normally shaped by beam shaping elements. As a result, divergent beams are directly shaped, and beams having approximately circular section can be generated with rarely generating aberration. As such elements, beam shaping elements whose both surfaces are cylindrical are proposed. For example, Japanese Patent Application Laid-Open No. 2002-208159 discloses such a beam shaping element.
In such beam shaping elements, high eccentric accuracy between generatrices on respective cylindrical surfaces is required (parallel eccentricity: about 1 to 10 μm tilt eccentricity: about 1 to 10 min.)
Further, in such beam shaping elements, alignment work with high accuracy is necessary at the time of incorporating a pickup, and thus the adjusting method becomes very difficult (parallel eccentricity: about 1 to 10 μm, tilt eccentricity: 1 to 5 min.)